1. Field of the Invention
The present invention relates to a speech coding apparatus in which pieces of speech information are coded to digital signals having a small information volume and the digital signals are transmitted and decoded to perform an efficient data transmission. Also, the present invention relates to a linear prediction coefficient analyzing apparatus in which a digital speech signal having an analyzing time-length is analyzed to obtain a linear prediction coefficient used in the speech coding apparatus. Also, the present invention relates to a noise reducing apparatus in which noise existing in speech information is reduced at a moderate degree before the speech information is coded in the speech coding apparatus.
2. Description of the Related Art
In a digital moving communication field such as a portable telephone, a compression coding method for speech signals transmitted at a low bit rate is required because subscribers in a digital moving communication have been increased, and research and development on the compression coding method have been carried out in various research facilities. In Japan, a coding method called a vector sum excited linear prediction (VSELP), proposed by the Motorola company, in which signals are transmitted at a bit rate of 11.2 kbits per second (kbps) is adopted as a standard coding method for a digital portable telephone. The digital portable telephone manufactured according to the VSELP coding method has been put on sale in Japan since the autumn of 1994. Also, another coding method called a pitch synchronous innovation code exited linear prediction (PSI-CELP), proposed by the NTT moving communication network Co., LTD., in which signals are transmitted at a bit rate of 5.6 kbps is adopted in Japan as a next standard coding method for a next portable telephone, and the development of the next portable telephone is going on now. These standard coding methods are obtained by improving a CELP which is disclosed by M. R. Schroeder in "High Quality Speech at Low Bit Rates" Proc. ICASSP, 85, pp.937-940. In this CELP coding method, speech information obtained from an input speech is separated into sound source information based on vibrational sounds of vocal cords and vocal tract information based on shapes of a vocal tract extending from the vocal cords to a mouth. The sound source information is coded according to a plurality of sound source samples stored in a code book while considering the vocal tract information and is compared with the input speech, and the vocal tract information is coded with a linear prediction coefficient. That is, an analysis by synthesis (A-b-S) method is adopted in the CELP coding method.